The present invention relates to a thermal ink jet printing apparatus.
A thermal ink jet printing apparatus has been proposed in various forms in the past. One of conventional ink jet printing apparatuses uses a film which is formed with a number of small apertures for retaining liquid ink therein. While such a film runs in contact with a thermal head, the ink is ejected from the film to form an image on a recording medium. Specifically, heating elements included in the thermal head are selectively energized by an electric signal representive of a particular image pattern. Heat generated by each energized heating element causes, among various components constituting the ink, the components having low boiling points to evaporate to thereby form a bubble. The ink is ejected from the apertures of the film by the pressure of the bubble to reach a recording medium which is spaced apart from the film by a small gap, thereby forming a spot on the recording medium. In this type of apparatus, when the apertures for forming a single ink spot corresponding to a single heating element are arranged in a density higher than the recording density, a single spot is formed by a plurality of ink drops having been ejected from nearby apertures. Hence, even when some of the apertures are sopped up and unable to eject ink therefrom, they do not have critical influence on the result of printing, i.e., a spot without the local omission of dots is achievable. This type of ink jet printing apparatus is disclosed in, for example, U.S. Pat. No. 4,608,577 (Hori).
However, the above-described type of conventional thermal ink jet printing apparatus has some problems left unsolved, as follows. Since a single ink spot corresponding to a single heating element is constituted by a plurality of ink drops ejected from the apertures of the film, it is difficult for the ink drops to cover the entire area of the spot due to the short amount of ink drops. More specifically, the ink spot is the collection of a plurality of physically separate pixels, resulting in low image density.
The film may be held in contact with a recording medium in the event of printing in order to prevent the sharpness of the ink spots from being lowered by the scattering of ink drops, as also proposed in the past. The prerequisite with this kind of scheme is that the front of the film and the walls of the apertures be repulsive to ink. In this case, however, the amount of ink on the recording medium is further reduced since the apertures of the film are not filled with ink.